Conformance improvement in a subterranean hydrocarbon-bearing formation using a crosslinked polymer

ABSTRACT

Conformance improvement is achieved in a subterranean hydrocarbon-bearing formation using a gel comprising a high molecular weight, water-soluble, carboxylate-containing polymer, a chromium III/carboxylate complex capable of crosslinking the polymer and an aqueous solvent. The gel components are combined at the surface and injected into the desired treatment zone via a wellbore to form a continuous single-phase gel.

This is a continuation-in-part application of copending application Ser.No. 822,709 filed on Jan. 27, 1986, now U.S. Pat. No. 4,683,949 which isa continuation-in-part application of application Ser. No. 807,416 filedon Dec. 10, 1985, now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field:

The invention relates to a process for reducing the permeability in arelatively high permeability region of a subterraneanhydrocarbon-bearing formation and more particularly to a process forimproving areal and vertical conformance and flow profiles at or awayfrom a production and/or injection wellbore penetrating thehydrocarbon-bearing formation.

2. Description of Related Art:

Poor vertical conformance results from the vertical juxtaposition ofrelatively high permeability geologic zones to relatively lowpermeability zones within a subterranean formation. Poor arealconformance results from the presence of high permeability streaks andhigh permeability anomalies within the formation matrix, such asvertical fractures and networks of the same, which have very highpermeability relative to the formation matrix. Fluids generally exhibitpoor flow profiles and sweep efficiencies in subterranean formationshaving poor vertical or areal conformance. Poor conformance isparticularly a problem where vertical heterogeneity and/or fracturenetworks or other structural anomalies are in fluid communication with asubterranean wellbore across which fluids are injected or produced.

A number of attempts to remedy conformance problems exist. U.S. Pat.Nos. 3,762,476; 3,981,363; 4,018,286; and 4,039,029 to Gall or Gall etal describe various processes wherein gel compositions are formed inhigh permeability zones of subterranean formations to reduce thepermeability therein. According to U.S. Pat. No. 3,762,476, a polymersuch as polyacrylamide is injected into a formation followedsequentially by a crosslinking agent. The sequentially injected slugsare believed to permeate the treatment zone of the formation and gel insitu.

It is generally held that effective polymer/crosslinking agent systemsnecessitate sequential injection of the gel components because gelsystems mixed on the surface often set up before they can effectivelypenetrate the treatment region. However, in practice, treatments such asthat disclosed in U.S. Pat. No. 3,762,476 using sequentially injectedgel systems have proven unsatisfactory because of the inability toachieve complete mixing and gelation in the formation. As a result, gelsonly form at the interface of the unmixed gel components and often inregions remote from the desired treatment region. A need exists for agelation process capable of forming gels having a predetermined gelationrate, strength, and stability to satisfy the particular demands of adesired treatment region in a subterranean hydrocarbon-bearingformation.

SUMMARY OF THE INVENTION

The present invention provides a process for improving vertical andareal conformance in a subterranean hydrocarbon-bearing formationpenetrated by a production and/or injection well and for correspondinglyimproving flow profiles and sweep efficiencies of injected and/orproduced fluids in the formation. The objectives are achieved by meansof a tailor-made flowing or non-flowing polymer gel.

The gel comprises a high molecular weight, carboxylate-containingpolymer and a chromic carboxylate crosslinking agent. The gel isprepared by forming a gelation solution above ground containing thepolymer and crosslinking agent and injecting the solution into thedesired treatment region via a wellbore in fluid communicationtherewith. The gelation solution is advantageously at least partiallygelled by the time it reaches the treatment region to inhibit or preventits propagation into adjoining regions where no treatment is desired.The final gel is a continuous single-phase gel which substantiallyreduces permeability in the treatment region.

After the gelation treatment, fluids may be injected into or producedfrom the hydrocarbon-bearing regions of the formation in the fluidcommunication with the wellbore. The gel is substantially incapable offlowing from the treatment region and is substantially permanent andresistant to in situ degradation.

The process provides distinct advantages over gelation processes knownin the art. The practitioner of the present invention customizes ortailors a gel to a specific subterranean application by firstdetermining the treatment demands of a desired subterranean region.Given these treatment demands, one can predetermine the gelation rateand resultant gel strength and stability which are required of a gel tomeet the demands. Thereafter, a gel having the required predeterminedproperties is produced under controlled conditions at the surface byutilizing observed correlations between specific controllable gelationparameters and resultant gel properties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in the context of specific termswhich are defined as follows. The formation consists of two generalregions, the "matrix" and "anomalies." An "anomaly" is a volume or voidspace in the formation having very high permeability relative to thematrix. It is inclusive of terms such as streaks, fractures, fracturenetworks, vugs, solution channels, caverns, washouts, cavities, etc. The"matrix" is substantially the remainder of the formation volumecharacterized as essentially homogeneous, continuous, sedimentaryreservoir material free of anomalies and often competent.

The matrix consists of horizontal "zones" of distinctive subterraneanmaterial of continuous geologic properties which extend in thehorizontal direction. "Vertical conformance" is a measure of the degreeof geologic uniformity in permeability as one moves vertically acrossthe formation. "Areal conformance" is a measure of the degree ofgeologic uniformity in permeability as one moves horizontally across theformation. A "flow profile" qualitatively describes the uniformity offluid flow through a subterranean formation while "sweep efficiency" isthe quantitative analog of "flow profile." "Plugging" is a substantialreduction in permeability in a region of a formation.

The term "gel" as used herein is directed to a continuousthree-dimensional crosslinked polymeric network having an ultra highmolecular weight. The gel contains a liquid medium such as water whichis confined within the solid polymeric network. The fusion of a liquidand a solid component into a single-phase system provides the gel with aunique phase behavior. Gels employed by the present invention havesufficient structure so as not to propagate from the confines of aplugged volume into a less permeable region of the formation adjoiningthe volume when injected into the volume.

The gel is qualitatively defined as "flowing" or "non-flowing" based onits ability to flow under the force of gravity when unconfined on thesurface at ambient atmospheric conditions. A flowing gel flows underthese conditions; a non-flowing gel does not. Nonetheless, both anon-flowing gel and a flowing gel are defined herein as havingsufficient structure so as not to propagate from the confines of thedesired treatment region into a less permeable adjoining region wheninjected into the subterranean treatment region.

"Partially gelled" solutions are also referred to herein. A partiallygelled solution is at least somewhat more viscous than an uncrosslinkedpolymer solution such that it is incapable of entering a less permeableregion where no treatment is desired, but sufficiently fluid such thatit is capable of displacement into a desired treatment zone. Thecrosslinking agent of the partially gelled solution has reactedincompletely with the polymer with the result that neither all of thepolymer nor all of the crosslinking agent in the gelation solution istotally consumed by the crosslinking reaction. The partially gelledsolution is capable of further crosslinking to completion resulting inthe desired gel without the addition of more crosslinking agent.

"Crosslinked to completion" means that the gel composition is incapableof further crosslinking because one or both of the required reactants inthe initial solution are consumed. Further crosslinking is only possibleif either polymer, crosslinking agent, or both are added to the gelcomposition.

The gel composition utilized in the present invention is comprised of acarboxylate-containing polymer and a crosslinking agent. Thecarboxylate-containing polymer may be any crosslinkable, high molecularweight, water-soluble, synthetic polymer or biopolymer containing one ormore carboxylate species. The average molecular weight of thecarboxylate-containing polymer is in the range of about 10,000 to about50,000,000 and preferably about 100,000 to about 20,000,000, and mostpreferably about 200,000 to about 15,000,000.

Biopolymers useful in the present invention include polysaccharides andmodified polysaccharides. Exemplary biopolymers are xanthan gum, guargum, carboxymethylcellulose, o-carboxychitosans, hydroxyethylcellulose,hydroxypropylcellulose, and modified starches. Useful synthetic polymersinclude inter alia acrylamide polymers, such as polyacrylamide,partially hydrolyzed polyacrylamide and terpolymers containingacrylamide, acrylate, and a third species. As defined herein,polyacrylamide (PA) is an acrylamide polymer having substantially lessthan 1% of the acrylamide groups in the form of carboxylate groups.Partially hydrolyzed polyacrylamide (PHPA) is an acrylamide polymerhaving at least 1%, but not 100%, of the acrylamide groups in the formof carboxylate groups. The acrylamide polymer may be prepared accordingto any conventional method known in the art, but preferably has thespecific properties of acrylamide polymer prepared according to themethod disclosed by U.S. Pat. No. Re. 32,114 to Argabright et alincorporated herein by reference.

The crosslinking agent is a chromic carboxylate complex. The term"complex" is defined herein as an ion or molecule containing two or moreinterassociated ionic, radical or molecular species. A complex ion as awhole has a distinct electrical charge while a complex molecule iselectrically neutral. The term "chromic carboxylate complex" encompassesa single complex, mixtures of complexes containing the same carboxylatespecies, and mixtures of complexes containing differing carboxylatespecies.

The complex of the present invention includes at least one or moreelectropositive chromium III species and one or more electronegativecarboxylate species. The complex may advantageously also contain one ormore electronegative hydroxide and/or oxygen species. It is believedthat, when two or more chromium III species are present in the complex,the oxygen or hydroxide species may help to bridge the chromium IIIspecies. Each complex optionally contains additional species which arenot essential to the polymer crosslinking function of the complex. Forexample, inorganic mono- and/or divalent ions, which function merely tobalance the electrical charge of the complex, or one or more watermolecules may be associated with each complex. Representative formulaeof such complexes include:

    [Cr.sub.3 (CH.sub.3 CO.sub.2).sub.6 (OH).sub.2 ].sup.+1 ;

    [Cr.sub.3 (OH).sub.2 (CH.sub.3 CO.sub.2).sub.6 ]NO.sub.3.6H.sub.2 O;

    [Cr.sub.3 (H.sub.2 O).sub.2 (CH.sub.3 CO.sub.2).sub.6 ].sup.+3 ;

    [Cr.sub.3 (H.sub.2 O).sub.2 (CH.sub.3 CO.sub.2).sub.6 ](CH.sub.3 CO.sub.2).sub.3.H.sub.2 O;

etc.

Trivalent chromium and chromic ion are equivalent terms encompassed bythe term chromium III species as used herein. The carboxylate speciesare advantageously derived from water-soluble salts of carboxylic acids,especially low molecular weight mono-basic acids. Carboxylate speciesderived from salts of formic, acetic, propionic, and lactic acid, lowersubstituted derivatives thereof and mixtures thereof are especiallypreferred. The carboxylate species include the following water-solublespecies: formate, acetate, propionate, lactate, lower substitutedderivatives thereof, and mixtures thereof. The optional inorganic ionsinclude sodium, sulfate, nitrate and chloride ions.

A host of complexes of the type described above and their method ofpreparation are well known in the leather tanning art. These complexesare described in Shuttleworth and Russel, Journal of the Society ofLeather Trades' Chemists, "The Kinetics of Chrome Tannage Part I.,"United Kingdom, 1965, v. 49, p. 133-154; "Part III.," United Kingdom,1965, v. 49, p. 251-260; "Part IV.," United Kingdom, 1965, v. 49, p.261-268; and Von Erdman, Das Leder, "Condensation of MononuclearChromium (III) Salts to Polynuclear Compounds," Eduard Roether Verlag,Darmstadt, Germany, 1963, v. 14, p. 249; and incorporated herein byreference. Udy, Marvin J., Chromium, Volume 1: Chemistry of Chromium andits Compounds, Reinhold Publishing Corp., N.Y., 1956, pp. 229-233; andCotton and Wilkinson, Advanced Inorganic Chemistry 3rd Ed., John Wiley &Sons, Inc., N.Y., 1972, pp. 836-839, further describe typical complexeswhich may be within the scope of the present invention and areincorporated herein by reference. The present invention is not limitedto the specific complexes and mixtures thereof described in thereferences, but may include others satisfying the above-stateddefinition.

The gel is formed by admixing the carboxylate-containing polymer and thecrosslinking agent at the surface to form an injectable gelationsolution. Surface admixing broadly encompasses inter alia mixing thesolution in bulk at the surface prior to injection or simultaneouslymixing the solution at or near the wellhead by in-line mixing meanswhile injecting it. Admixing is accomplished for example by dissolvingthe starting materials for the crosslinking agent in an appropriateaqueous solvent. Exemplary starting materials include solid CrAc₃. H₂ O,solid Cr₃ Ac₇ (OH)₂ or a solution labeled "Chromic Acetate 50% Solution"commercially available, for example, from McGean Chemical Co., Inc.,1250 Terminal Tower, Cleveland, Ohio 44113, U.S.A. The crosslinkingagent solution is then mixed with an aqueous polymer solution to producethe gelation solution. Among other alternatives, the starting materialsfor the crosslinking agent can be dissolved directly in the aqueouspolymer solution to form the gelation solution in a single step.

The aqueous solvent of the gelation solution may be fresh water or abrine having a total dissolved solids concentration up to the solubilitylimit of the solids in water. Inert fillers such as crushed or naturallyfine rock material or glass beads can also be added to the gelationsolution to reinforce the gel network structure.

The present process enables the practitioner to customize or tailor-makea gel having a predetermined gelation rate and predetermined gelproperties of strength and stability from the above-describedcomposition. The gelation rate is defined as the degree of gel formationas a function of time or, synonymously, the rate of crosslinking in thegelation solution. The degree of crosslinking may be quantified in termsof gel viscosity and/or strength. Gel strength of a non-flowing gel isdefined as the coherence of the gel network or resistance to deformationunder external forces. Gel strength of a flowing gel is defined as theresistance of the gel to filtration or flow. Stability is defined aseither thermal or phase stability. Thermal stability is the ability of agel to withstand temperature extremes without degradation. Phasestability is the ability of a gel to resist syneresis which can detractfrom the gel structure and performance.

Tailor-making or customizing a gel in the manner of the presentinvention to meet the demands of a particular treatment region isprovided in part by correlating the independent gelation parameters withthe dependent variables of gelation rate and resultant gel strength andstability. The independent gelation parameters are the surface and insitu gelation conditions including: temperature, pH, ionic strength andspecific electrolytic makeup of the solvent, polymer concentration,ratio of the weight of polymer to the combined weight of chromium IIIand carboxylate species in the mixture, degree of polymer hydrolysis,and average molecular weight of the polymer.

The operable ranges of the gelation parameters are correlated with thedependent variables of gelation rate and resultant gel properties bymeans including qualitative bottle testing, quantitative viscosimetricanalysis, packed-column flooding, and core flooding. The operable rangesof a number of gelation parameters and their correlation with thedependent variables are described below.

The lower temperature limit of the gelation solution at the surface isthe freezing point of the solution and the upper limit is essentiallythe thermal stability limit of the polymer. The solution is generallymaintained at ambient temperature or higher at the surface. Thetemperature may be adjusted by heating or cooling the aqueous solvent.Increasing the temperature within the prescribed range increases thegelation rate.

The initial pH of the gelation solution is within a range of about 3 to13 and preferably about 6 to 13. Although gelation can occur at anacidic pH, lowering the initial pH of the solution below 7 does notfavor gelation. The initial pH of the solution is most preferablyalkaline, i.e., greater than 7 to about 13. Increasing the pH within theprescribed range increases the rate of gelation.

The polymer concentration in the solution is about 500 ppm up to thesolubility limit of the polymer in the solvent or the rheologicalconstraints of the polymer solution, preferably about 1000 to about200,000 ppm, and most preferably about 3000 to about 100,000. Increasingthe polymer concentration increases the gelation rate and ultimate gelstrength at a constant ratio of polymer to crosslinking agent.

The ionic strength of the solvent can be from that of deionizeddistilled water to that of a brine having an ion concentrationapproaching the solubility limit of the brine. Increasing the ionicstrength of the solution can increase the gelation rate.

The weight ratio of polymer to chromium III and carboxylate speciescomprising the mixture is about 1:1 to about 500:1, preferably about2.5:1 to about 100:1, and most preferably about 5:1 to about 40:1.Decreasing the ratio generally increases the gelation rate and up to acertain point generally increases the gel strength, especially at aconstant high polymer concentration.

When an acrylamide polymer is employed, the degree of hydrolysis isabout 0 to 60% and preferably about 0 to 30%. Within the preferredrange, increasing the degree of hydrolysis increases the gelation rate.Increasing the molecular weight of the polymer increases the gelstrength.

It is apparent from these correlations that one can produce gels acrossa very broad range of gelation rates and gel properties as a function ofthe gelation conditions. Thus, to effect an optimum gelation treatmentaccording to the present process, the practitioner predetermines thegelation rate and properties of the resultant gel which meet thetreatment demands of the given region and thereafter produces the gelhaving these predetermined characteristics. The treatment demandsinclude the in situ gelation conditions such as temperature, connatewater properties, and permeability of the region as well as the posttreatment conditions such as injection and production pressures.Analytical methods known to one skilled in the art are used to determinethe treatment demands. The treatment demands provide criteria topredetermine the gelation rate and resultant gel properties in themanner described above and continuing hereafter.

The gelation rate is advantageously sufficiently slow to enablepreparation of the gelation solution at the surface, injection of thesolution as a uniform slug into the wellbore, and displacement of theentire solution into the desired treatment zone. Too rapid a gelationrate produces excessive gelation of the solution at the surface whichresults in a solution that may be difficult, if not impossible, toinject into the wellbore or formation due to its rheological properties.At the same time, the gelation rate must be sufficiently rapid to enablecompletion of the reaction within a reasonable period of time so thatthe well may be returned to injection or production after treatment.

When treating anomalies, at least partial gelation of the solution, ifnot complete gelation for some flowing gels, is advantageous before thesolution reaches the face bounding the matrix and the anomaly to preventthe solution from penetrating the matrix as well as the anomaly.Substantial penetration of the matrix by the solution and the ensuingpermeability reduction of the matrix are counterproductive to theplugging of anomalies. The values of the independent variables in theprocess are carefully selected to achieve a gelation rate meeting thesecriteria.

The volume of solution injected into the formation is a function of thevolume and location of the desired treatment region and the degree ofpenetration into the treatment region by the solution. One skilled inthe art can determine the required volume of gel for a given treatmentregion. Placement of the gelation solution in the treatment region maybe facilitated by zone isolation means such as packers and the like.

The injection rate is a function of the gelation rate and operationalconstraints of injection pressure and pumping limits. The requiredinjection rate is fixed such that all of the solution can be practicallyinjected into the treatment zone before it becomes unpumpable. Thegelation time of the gel ranges from near instantaneous for flowing gelsup to 48 hours or longer for both flowing and non-flowing gels. Longergelation times are limited by practical considerations of lostproduction when injection and production wells are shut in.

The process is applicable to conformance treatment of formations undermost conditions and is specific to treating regions within the formationwhich are in fluid communication with an injection or production well.The flowing gel is especially applicable to the treatment of anomaliessuch as streaks of relatively high permeability, fractures or fracturenetworks in direct communication via the anomaly with an injection wellbut not also in direct communication via the anomaly with a productionwell. The ultimate gel is termed a flowing gel as defined herein becauseit would flow if unconfined on the surface. However, the flowing gel issufficiently crosslinked to remain in place under injection conditionsin the anomaly when confined thereby. Thus, the flowing gel is capableof effectively plugging the anomaly.

The flowing gel is not generally suitable for treatment of anomalies indirect communication via the anomaly with production wells becauseflowing gels do not have sufficient strength to withstand the drawdownpressure during production and may flow back into the wellbore. Fortreatment of anomalies in direct communication with production wells,non-flowing rigid gels having sufficient strength to withstand theproduction drawdown pressure are preferred. It is preferred thatsubstantially none of the gel flows back into the wellbore when oil isproduced after the conformance treatment.

In some specialized cases, the degree of partial gelation of theinjected solution can be maintained sufficiently low to enable thesolution to enter a selected high permeability zone of the matrix andcrosslink to completion in situ as either a non-flowing gel or a flowinggel. Both flowing and non-flowing gels can be used for treatment of highpermeability zones of the matrix because in general neither will flowfrom the treatment zone upon complete gelation, a necessary conditionfor the present invention. However, non-flowing gels are preferred fortreatment of high permeability zones in direct communication withproduction wells because of their increased strength.

Gels having a predetermined gelation rate and resultant gel propertiesto meet the treatment demands of a given region are produced byadjusting and setting the surface gelation conditions as they correlateto the gelation rate and gel properties. Accordingly the gels areproduced in a manner which renders them insensitive to most extremeformation conditions. The gels can be applied to the treatment of manydifferent geological structures including high permeability zones withinthe formation matrix and anomalies external to the matrix such asfractures and other cavities. The gels can be stable at formationtemperatures as high as 115° C. and at any formation pH contemplated.The gels are relatively insensitive to the stratigraphy of the rock andcan be employed in carbonate and sandstone strata and unconsolidated orconsolidated strata having varying mineralogy. Once the gels are inplace, it is extremely difficult to displace the gels by physical orchemical means other than total destruction of the crosslinked network.The gels may be reversible on contact with hydrogen peroxide or sodiumhypochlorite, but are substantially insoluble in the formation fluids.

The following examples demonstrate the practice and utility of thepresent invention but are not to be construed as limiting the scopethereof.

EXAMPLES

The examples are formatted as a table of data which describe theformulation and maturation of several gels. Each gel is represented inthe table by a single horizontal entry. Data include the conditions forproducing the gel and the qualitative strength of the produced gel.

The following gel strength code and nomenclature are useful forinterpreting the table.

GEL STRENGTH CODE

A: No detectable gel formed: the bulk of the solution appears to havethe same viscosity as the original polymer solution, although isolatedlocal gel balls may be present.

B: Highly flowing gel: the gel appears to be only slightly more viscousthan the initial polymer solution.

C: Flowing gel: most of the gel flows to the bottle cap by gravity uponinversion.

D: Moderately flowing gel: only a small portion (5-10%) of the gel doesnot readily flow to the bottle cap by gravity upon inversion (usuallycharacterized as a tonguing gel).

E: Barely flowing gel: the gel can barely flow to the bottle cap and/ora significant portion (>15%) of the gel does not flow by gravity uponinversion.

F: Highly deformable nonflowing gel: the gel does not flow to the bottlecap by gravity upon inversion.

G: Moderately deformable nonflowing gel: the gel deforms about half waydown the bottle by gravity upon inversion.

H: Slightly deformable nonflowing gel: only the gel surface slightlydeforms by gravity upon inversion.

I: Rigid gel: there is no gel surface deformation by gravity uponinversion.

J: Ringing rigid gel: a tuning fork-like mechanical vibration can befelt upon tapping the bottle.

NOMENCLATURE

Polymer: chemical name of polymer

Polymer MW: average molecular weight of the polymer

Polymer Conc: polymer concentration in the polymer solution (ppm)

Polymer Solvent: aqueous solvent of the polymer solution (Fresh=DenverTap Water; Brine=5000 ppm NaCl, 200 ppm CaCl₂)

Weight Ratio Polymer: Ions: weight ratio of polymer to crosslinkingagent in the gelation solution

Time: gelation time (days)

Gel Code: gel strength code.

The polymer solutions of the following examples are prepared by dilutingaqueous polymer solutions with an aqueous solvent. The dilute polymersolution is combined with the crosslinking agent solution in a widemouthbottle to form a sample. The sample is gelled in the capped bottle at50° C. and the qualitative gel strength is determined by periodicallyinverting the bottle. In all of the examples, the crosslinking agentsolution is that of the present invention (i.e., a complex or mixture ofcomplexes comprising chromium III and acetate ions).

                                      TABLE OF EXAMPLES                           __________________________________________________________________________    Run         Polymer                                                                              Polymer                                                                            Polymer                                                                            Weight Ratio                                                                             Gel                                   No.                                                                              Polymer  MW     Conc.                                                                              Solvent                                                                            Polymer:Ions                                                                         Time                                                                              Code                                  __________________________________________________________________________    1  PHPA-1   11,000,000                                                                           5000 Fresh                                                                              10     7   D                                     2  PHPA-1   11,000,000                                                                           5000 Fresh                                                                              20     7   C                                     3  PHPA-2   3,000,000                                                                            5000 Fresh                                                                              10     7   A                                     4  PHPA-2   3,000,000                                                                            7500 Fresh                                                                              10     7   C                                     5  Guar Gum 500,000                                                                              5000 Fresh                                                                              20     24  A                                     6  Guar Gum 500,000                                                                              10,000                                                                             Fresh                                                                              20     17  B                                     7  Poly (methylvinyl                                                                       80,000                                                                              5000 Fresh                                                                              10     14  A                                        ether/maleic                                                                  anhydride)                                                                 8  Poly (methylvinyl                                                                       80,000                                                                              10,000                                                                             Fresh                                                                              40     20  B                                        ether/maleic                                                                  anhydride)                                                                 9  Xanthan Gum                                                                            1,500,000                                                                            5000 Fresh                                                                              10     5   A                                     10 Xanthan Gum                                                                            1,500,000                                                                            8000 Fresh                                                                              10     12  B                                     11 Xanthan Gum                                                                            1,500,000                                                                            5000 Brine                                                                              10     12  C                                     12 Xanthan Gum                                                                            1,500,000                                                                            8000 Brine                                                                              10     12  C                                     13 Carboxymethyl-                                                                         700,000                                                                              5000 Brine                                                                              20     0.2 F                                        cellulose                                                                  14 Carboxymethyl-                                                                         700,000                                                                              5000 Brine                                                                              10     0.2 F                                        cellulose                                                                  15 Carboxymethyl-                                                                         700,000                                                                              5000 Fresh                                                                              20     0.2 F                                        cellulose                                                                  16 Carboxymethyl-                                                                         700,000                                                                              5000 Fresh                                                                              10     0.2 F                                     __________________________________________________________________________

PHPA-1 is produced according to the process of U.S. Pat. No. Re. 32,114while PHPA-2 is not. Both polymers are 30% hydrolyzed.

The data of the Table show that gels can be produced from a number ofcarboxylate-containing polymers whether synthetic polymers orbiopolymers. Although the strength of the gels appears weak in manycases, the gels can be strengthened by increasing the reactantconcentrations. The data indicate that gel strength is also favored byincreased molecular weight of the polymer and choice of solvent.

The examples of copending parent application, Ser. No. 822,709, filed onJan. 27, 1986, are incorporated herein by reference.

While foregoing preferred embodiments of the invention have beendescribed and shown, it is understood that all alternatives andmodifications, such as those suggested and others, may be made theretoand fall within the scope of the invention.

We claim:
 1. A process for substantially plugging at least onerelatively high permeability region bounded by at least one relativelylow permeability region in a hydrocarbon-bearing formation below anearthen surface, said formation penetrated by a wellbore in fluidcommunication with said at least one relatively high permeabilityregion, the process consisting essentially of the steps of:(a) preparinga gelation solution at the surface consisting essentially of awater-soluble carboxylate-containing polymer, a complex capable ofcrosslinking said polymer and formed of at least one electropositivechromium III species and at least one electronegative carboxylatespecies, and a solvent for said polymer and said complex; (b) injectingsaid gelation solution into said wellbore; (c) displacing said gelationsolution into said at least one relatively high permeability region; and(d) crosslinking said gelation solution substantially to completion insaid at least one relatively high permeability region to form acrosslinked gel which substantially plugs said at least one relativelyhigh permeability region.
 2. The process of claim 1 wherein saidgelation solution is partially gelled upon injection such that saidgelation solution is sufficiently flowing to penetrate said at least onerelatively high permeability region but is sufficiently gelled toprevent substantial penetration of said at least one relatively lowpermeability region.
 3. The process of claim 1 wherein said carboxylatespecies is selected from the group consisting of formate, acetate,propionate, lactate, lower substituted derivatives thereof, and mixturesthereof.
 4. The process of claim 1 wherein said gelation solution issubstantially ungelled upon injection.
 5. The process of claim 1 whereinsaid at least one relatively high permeability region is an anomaly andsaid at least one relatively low permeability region is a matrix.
 6. Theprocess of claim 1 wherein said at least one relatively highpermeability region is a first zone of a matrix and said at least onerelatively low permeability region is a second zone of a matrix.
 7. Theprocess of claim 1 wherein said carboxylate-containing polymer is abiopolymer.
 8. The process of claim 1 wherein saidcarboxylate-containing polymer is an acrylamide polymer having anaverage molecular weight of about 2 million to about 10 million and amolecular weight distribution of about 0.02 to about 0.22 as determinedby the ratio of σ/M wherein σ is the standard deviation of molecularweight of abundance of a molecular species present in the polymer and Mis the average molecular weight of the polymer.
 9. A process forsubstantially plugging at least one relatively high permeability regionbounded by at least one relatively low permeability region in ahydrocarbon-bearing formation below an earthen surface, said formationpenetrated by a wellbore in fluid communication with said at least onerelatively high permeability region, the process consisting essentiallyof the steps of:(a) preparing a gelation solution at the surfaceconsisting essentially of a water-soluble carboxylate-containingpolymer, a complex capable of crosslinking said polymer and formed of atleast one electropositive chromium III species, at least oneelectronegative carboxylate species, and at least one species selectedfrom the group consisting of electronegative oxygen species,electronegative hydroxide species, inorganic monovalent ions, inorganicdivalent ions, water molecules, and mixtures thereof, and a solvent forsaid polymer and said complex; (b) injecting said gelation solution intosaid wellbore; (c) displacing said gelation solution into said at leastone relatively high permeability region; and (d) crosslinking saidgelation solution substantially to completion in said at least onerelatively high permeability region to form a crosslinked gel whichsubstantially plugs said at least one relatively high permeabilityregion.
 10. The process of claim 9 wherein said gelation solution ispartially gelled upon injection such that said gelation solution issufficiently flowing to penetrate said at least one relatively highpermeability region but is sufficiently gelled to prevent substantialpenetration of said at least one relatively low permeability region. 11.The process of claim 9 wherein said carboxylate species is selected fromthe group consisting of formate, acetate, propionate, lactate, lowersubstituted derivatives thereof, and mixtures thereof.
 12. The processof claim 9 wherein said gelation solution is substantially ungelled uponinjection.
 13. The process of claim 9 wherein said at least onerelatively high permeability region is an anomaly and said at least onerelatively low permeability region is a matrix.
 14. The process of claim9 wherein said at least one relatively high permeability region is afirst zone of a matrix and said at least one relatively low permeabilityregion is a second zone of a matrix.
 15. The process of claim 9 whereinsaid carboxylate-containing polymer is a biopolymer.
 16. The process ofclaim 9 wherein said carboxylate-containing polymer is an acrylamidepolymer having an average molecular weight of about 2 million to about10 million and a molecular weight distribution of about 0.02 to about0.22 as determined by the ratio of σ/M wherein σ is the standarddeviation of molecular weight of abundance of a molecular speciespresent in the polymer and M is the average molecular weight of thepolymer.
 17. A process for substantially reducing the permeability of atleast one relatively high permeability region bounded by at least onerelatively low permeability region in a hydrocarbon-bearing formationmatrix below an earthen surface penetrated by a wellbore in fluidcommunication with said at least one relatively high permeabilityregion, the process consisting essentially of the steps of:(a) preparinga gelation solution at the surface consisting essentially of awater-soluble carboxylate-containing polymer, a complex capable ofcrosslinking said polymer and formed of at least one electropositivechromium III species and at least one electronegative carboxylatespecies, and a solvent for said polymer and said complex; (b)crosslinking said gelation solution substantially to completion at saidearthen surface to form an injectable crosslinked flowing gel; (c)injecting said gel into said wellbore; and (d) displacing said gel intosaid at least one relatively high permeability region to substantiallyplug said at least one relatively high permeability region.
 18. Theprocess of claim 17 wherein said carboxylate species is selected fromthe group consisting of formate, acetate, propionate, lactate, lowersubstituted derivatives thereof, and mixtures thereof.
 19. The processof claim 17 wherein said at least one relatively high permeabilityregion is an anomaly and said at least one relatively low permeabilityregion is a matrix.
 20. The process of claim 17 wherein said at leastone relatively high permeability region is a first zone of a matrix andsaid at least one relatively low permeability region is a second zone ofa matrix.
 21. The process of claim 17 wherein saidcarboxylate-containing polymer is a biopolymer.
 22. The process of claim17 wherein said carboxylate-containing polymer is an acrylamide polymerhaving an average molecular weight of about 2 million to about 10million and a molecular weight distribution of about 0.02 to about 0.22as determined by the ratio of σ/M wherein σ is the standard deviation ofmolecular weight of abundance of a molecular species present in thepolymer and M is the average molecular weight of the polymer.
 23. Aprocess for substantially reducing the permeability of at least onerelatively high permeability region bounded by a relatively lowpermeability region in a hydrocarbon-bearing formation matrix below anearthen surface penetrated by a wellbore in fluid communication withsaid at least one relatively high permeability region, the processconsisting essentially of the steps of:(a) preparing a gelation solutionat the surface consisting essentially of a water-solublecarboxylate-containing polymer, a complex capable of crosslinking saidpolymer, said complex formed of at least one electropositive chromiumIII species, at least one electronegative carboxylate species, and atleast one species selected from the group consisting of electronegativeoxygen species, electronegative hydroxide species, inorganic monovalentions, inorganic divalent ions, water molecules, and mixtures thereof anda solvent for said polymer and said complex; (b) crosslinking saidgelation solution substantially to completion at said earthen surface toform an injectable crosslinked flowing gel; (c) injecting said gel intosaid wellbore; and (d) displacing said gel into said at least onerelatively high permeability region to substantially plug said at leastone relatively high permeability region.
 24. The process of claim 23wherein said carboxylate species is selected from the group consistingof formate, acetate, propionate, lactate, lower substituted derivativesthereof, and mixtures thereof.
 25. The process of claim 23 wherein saidat least one relatively high permeability region is an anomaly and saidat least one relatively low permeability region is a matrix.
 26. Theprocess of claim 23 wherein said at least one relatively highpermeability region is a first zone of a matrix and said at least onerelatively low permeability region is a second zone of a matrix.
 27. Theprocess of claim 23 wherein said carboxylate-containing polymer is abiopolymer.
 28. The process of claim 23 wherein saidcarboxylate-containing polymer is an acrylamide polymer having anaverage molecular weight of about 2 million to about 10 million and amolecular weight distribution of about 0.02 to about 0.22 as determinedby the ratio of σ/M wherein σ is the standard deviation of molecularweight of abundance of a molecular species present in the polymer and Mis the average molecular weight of the polymer.
 29. A process forsubstantially plugging at least one relatively high permeability regionbounded by at least one relatively low permeability region in ahydrocarbon-bearing formation below an earthen surface, said formationpenetrated by a wellbore in fluid communication with said at least onerelatively high permeability region, the process consisting essentiallyof the steps of:(a) preparing a gelation solution at the surfaceconsisting essentially of a water-soluble carboxylate-containingpolymer, a complex capable of crosslinking said polymer and formed of atleast one electropositive chromium III species and at least oneelectronegative acetate species, and a solvent for said polymer and saidcomplex; (b) injecting said gelation solution into said wellbore; (c)displacing said gelation solution into said at least one relatively highpermeability region; and (d) crosslinking said gelation solutionsubstantially to completion in said at least one relatively highpermeability region to form a crosslinked gel which substantially plugssaid at least one relatively high permeability region.
 30. A process forsubstantially plugging at least one relatively high permeability regionbounded by at least one relatively low permeability region in ahydrocarbon-bearing formation below an earthen surface, said formationpenetrated by a wellbore in fluid communication with said at least onerelatively high permeability region, the process consisting essentiallyof the steps of:(a) preparing a gelation solution at the surfaceconsisting essentially of a water-soluble carboxylate-containingpolymer, a complex capable of crosslinking said polymer and formed of atleast one electropositive chromium III species, at least oneelectronegative acetate species, and at least one species selected fromthe group consisting of electronegative oxygen species, electronegativehydroxide species, inorganic monovalent ions, inorganic divalent ions,water molecules, and mixtures thereof, and a solvent for said polymerand said complex; (b) injecting said gelation solution into saidwellbore; (c) displacing said gelation solution into said at least onerelatively high permeability region; and (d) crosslinking said gelationsolution substantially to completion in said at least one relativelyhigh permeability region to form a crosslinked gel which substantiallyplugs said at least one relatively high permeability region.
 31. Aprocess for substantially reducing the permeability of at least onerelatively high permeability region bounded by at least one relativelylow permeability region in a hydrocarbon-bearing formation matrix belowan earthen surface penetrated by a wellbore in fluid communication withsaid at least one relatively high permeability region, the processconsisting essentially of the steps of:(a) preparing a gelation solutionat the surface consisting essentially of a water-solublecarboxylate-containing polymer, a complex capable of crosslinking saidpolymer and formed of at least one electropositive chromium III speciesand at least one electronegative acetate species, and a solvent for saidpolymer and said complex; (b) crosslinking said gelation solutionsubstantially to completion at said earthen surface to form aninjectable crosslinked flowing gel; (c) injecting said gel into saidwellbore; and (d) displacing said gel into said at least one relativelyhigh permeability region to substantially plug said at least onerelatively high permeability region.
 32. A process for substantiallyreducing the permeability of at least one relatively high permeabilityregion bounded by a relatively low permeability region in ahydrocarbon-bearing formation matrix below an earthen surface penetratedby a wellbore in fluid communication with said at least one relativelyhigh permeability region, the process consisting essentially of thesteps of:(a) preparing a gelation solution at the surface consistingessentially of a water-soluble carboxylate-containing polymer, a complexcapable of crosslinking said polymer, said complex formed of at leastone electropositive chromium III species, at least one electronegativeacetate species, and at least one species selected from the groupconsisting of electronegative oxygen species, electronegative hydroxidespecies, inorganic monovalent ions, inorganic divalent ions, watermolecules, and mixtures thereof and a solvent for said polymer and saidcomplex; (b) crosslinking said gelation solution substantially tocompletion at said earthen surface to form an injectable crosslinkedflowing gel; (c) injecting said gel into said wellbore; and (d)displacing said gel into said at least one relatively high permeabilityregion to substantially plug said at least one relatively highpermeability region.